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a level chemistry > Chapter 25 - Arenes > Flashcards

Chapter 25 - Arenes Flashcards

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AP® Chemistry flashcards Periodic Table flashcards
1
Q

Molecular formula of benzene?

A

C6H6

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2
Q

Any compound containing a benzene ring is a ?

A

Aromatic compound/ arene

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3
Q

What shape is the benzene ring?

A

Hexagonal

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4
Q

Where are the delocalised electrons in benzene?

A

In the Pi bond , moving above and below the hexagonal ring,

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5
Q

How are the delocalised electrons shown in benzene?

A

Circle within hexagon

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6
Q

What is the C-C-H bond angle in benzene?

A

120

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7
Q

What is the shape of a benzene molecule?

A

Planar - trigonal planar

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8
Q

Benzene is an unsaturated molecule this means

A

It doesn’t only contain single bonds

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9
Q

The benzene ring is named as phenyl when it’s?

A

Branched

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10
Q

differences between the electrophilic addition and electrophilic substitution?

A

EA vs ES:
milder reagents vs catalyst
more reactive vs less reactive
easier to substitute the Br2 across the c=c in EA but harder to substitute in benzene

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11
Q

Why is electrophilic addition more reactive and ES?

A
  • Pi e-s are localised between the C=C -> induces a dipole in EA,
  • but in ES, Pi electrons are delocalised over the ring - doesn’t have a high enough electron density to induce a dipole in the Br2.
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12
Q

% yield equation?

A

actual MOLES / theoretical moles x 100

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13
Q

Actual moles equation?

A

% yield x TM / 100

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14
Q

Tm equation?

A

actual moles x 100 / % yield

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15
Q

benzene only reacts w bromine if a ? is present.

A

catalyst. Alkenes don’t need a catalyst to be able to react w bromine tho

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16
Q

Since a catalyst is needed for benzene to react w bromine?

A

benzene doesn’t decolourise B water so it’s not a normal alkene

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17
Q

The C-C bond in benzene?

A
  • all bonds are of equal length - 0.140

- the bond length is between that of a double and single bond

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18
Q

what is hydrogenation?

A

the addition of hydrogen

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19
Q

How much more stable is benzene than if it actually contained 3 ordinary C=C or structured lik previously expected?

A

It’s (360 -208) = 152 kJmol-1 more stable

* 360 comes from (3 x-120) which is the energy required to substitute 3H2s

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20
Q

how do we know double bonds don’t exist in benzene?

A

the hydrogenation of benzene can be measured experimentally and has been found to be -208 NOT -360 as would be expected if double bonds were present.

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21
Q

benzene structure?

A
  • 6 Pi electrons delocalised along the ring, this stabilises the structure.
  • all 6 p orbitals overlap SIDEWAYS
  • Pi bonding
  • one ring of 6 delocalised e-s
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22
Q

Evidence for benzene’s structure?

A
  • C-C bond lengths are equal in the delocalised system
  • The delocalised system reacts by substitution rather than addition
  • hydrogenation energies are lower than expected bc benzene is more stable/less reactive than a ring structure w 3 C=C
  • a benzene ring does not contain only single bonds so it’s unsaturated
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23
Q

Fridel Crafts reactions are?

A

just ES reactions. This reaction allows an alkyl group or an acyl group to be substituted onto a benzene ring.

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24
Q

Acyl group?

A

e. g. methanoyl C= Oxygen single bond H
e. g. ethanoyl CH3C=O
e. g. propanoyl CH3CH2C=O

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25
Q

⭐ IT’S REALLY IMPORTANT WHICH?

A

atom carries the + charge as this is the one that attaches to the benzene ring

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26
Q

if a compound contains a benzene ring?

A

it belongs to the homologous series arenes

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27
Q

what is an electrophile ?

A

a species w a + charge/ partial + charge that can accept a pair of e-s

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28
Q

in a substitution reaction?

A

a H is swapped on the benzene ring for another gr

29
Q

Nitration of benzene forms?

A

nitrobenzene + H2O

30
Q

Conditions for nitration of benzene?

A
  • conc H2SO4 catalyst

- 50-60 degrees C - higher temp would result in further subs

31
Q

reagents for nitration of benzene ?

A

conc HNO3

32
Q

3 steps for ES?

A

1) form electrophile
2) curly arrows (2)
3) reforming catalyst

33
Q

step 1 of nitration of benzene ?

A

HNO3 + H2SO4 -> NO2+ + H2O + HSO4-

34
Q

what is the temp needed for the nitration of benzene?

A

room temp

35
Q

how to work out what the electrophile is?

A

whatever gets added to the benzene ring

36
Q

be careful of?

A

where the arrows start - must be accurate

37
Q

halogenation of arenes conditions?

A

halogen carrier e.g. FeBr3

38
Q

what are friedel- Crafts reactions?

A
  • just ES

- allows an alkyl group/ acyl group to be subs onto benzene ring

39
Q

how to form a acyl + electrophile?

A
  • add Br to acyl group

- carry Br away w halogen carrier

40
Q

alkylation/ acylation of a benzene ring condition?

A

halogen carrier

41
Q

what is an acyl group?

A

group bonded to c=o

42
Q

why do u getting multiple subs with Friedel Crafts Reactions ?

A

once 1 alkyl/ acyl gr attached to BR, it becomes activated meaning it will react by ES again so that more groups are added

43
Q

bonding within phenol?

A

H

44
Q

Phenol is a ? at room temp

A

solid

45
Q

phenol solubility?

A
  • not soluble in cold water
  • soluble in hot water
  • as temp inc, the higher energy used to break H bonds
    H bonds strongest type of IMF so lots of energy required to break
46
Q

solubility steps?

A
  1. break IMFS
  2. Form H bonds with molecules
    ‘like dissolves like’
47
Q

pH of a warm solution of phenol?

A

5/6
- C6h5OH H2O on reversible arrow C6H5O- + H+
phenoxide ion formed

48
Q

phenol is a ? acid?

A
  • V weak acid

- reacts like typical weak acid almost - but can react like an acid or an arene

49
Q

phenol reacting as an acid: with sodium

A

observations: gas bubbles/ fizzing, metals will dissolve and disappear
equation: 2C6H5OH + 2Na -> 2C6H5O-Na+ (sodium phenoxide) + H2

50
Q

phenol reacting as an acid: with NaOH

A

C6H5OH + NaOH ➡ C6H5O-Na+ (sodium phenoxide) + H2O
weak acidic solution

add 1M of 0.5 NaOH to spatula of phenol

51
Q

phenol reacting as an acid: conc HCl + sodium phenoxide

A

phenol reformed w NaCl as other product

52
Q

phenol reacting as an acid: CuO

A

observations: black solid will get used up and blue solution will form
eq: 2 phenol + CuO -> H2O + (C6H5O)2Cu (copper (II) phenoxide)

53
Q

phenol reacting as an acid: w NaCO3 (anhydrous)

A

no reaction

phenol is too weak an acid to release CO2 from a carbonate

54
Q

ES of the BENZENE RING in phenol: Br2 (aq)

A

O: bromine water decolourised and WHITE PRECIPITATE FORMED
E: phenol + 3Br2 (aq) -> tribromophenol (?) + 3HBr

55
Q

ES of the BENZENE RING in phenol: nitric acid?

A

dilute acid and no catalyst
O: orange precipitate forms on gentle heating
E: phenol + HNO3 -> 2-nitrophenol or 4-nitrophenol + H2O

56
Q

ES of the BENZENE RING in phenol: nitric acid why

A
  • 2-nitrophenol is the major product (get 2x as much of it)
  • to get 2-nitophenol we can have NO2 on either 2 or 6 creates 2-
  • but for 4- can only have NO2 on position 4
57
Q

ES of the BENZENE RING in phenol: what type of reagents are involved?

A

dilute reagent + no catalyst

58
Q

why is the benzene ring more reactive in phenol than in benzene?

A
  • no halogen carrier needed like w benzene
  • bc OH group attached directly yo the benzene ring
  • this activates the ring causing phenol to be more reactive than benzene
59
Q

reactivity of phenol?

A
  • an electron pair (only one/2 of the lone pairs is at the right angle to b donated) from a p orbital on the O of the OH is donated to the benzene ring of phenol
  • this increases the electron density in the ring and activates it
  • phenol’s higher e- density allows it to induce a dipole in a halogen like Br2
    phenol reacts more readily w electrophiles than benzene
60
Q

Directing effect of OH group?

A
  • OH group is activating (e- donating), has more effect on some positions than others
  • OH group is 2,4 directing
    • 2x as much sub in position 2 than 4
61
Q

Directing effect of NH2 group?

A
  • The NH2 group is electron donating & so actibvating
    2,4 directing
  • 2x as much sub in position 2 than 4
62
Q

Directing effect of NO2 group?

A

NO2 groups are electron withdrawing & have 3- directing effect
- it takes e-s out of the benzene ring, makes it harder to substitute onto the group

63
Q

dissociation of phenol?

A

C6H5OH reversible arrow H+ + C6H5O-

phenol does not form many of these ion.

64
Q

why does phenol not form many of the phenoxide ion?

A
  • O is electronegative and so attracts the - charge (e-s)
  • this attracts the H+ ions
  • so more phenol molecules and less H+ formed
65
Q

describe the structure of benzene?

A
  • benzene has a trigonal planar shape w a bond angle of 120

- the C atoms are arranged in a hexagonal shape w the ring of delocalised e-s above and below the ring

66
Q

describe the bonding in benzene?

A
  • both sigma and pi bonds present
  • sigma bond formed by the direct overlap of sp2 orbitals
  • pi bond formed by sideways overlap of p orbitals
  • forming a ring of 6 delocalised e-s above and below benzene ring
    DRAW A DIAGRAM
67
Q

Benzoic acid reacts like ?????? in nitration reaction?

A

benzene

68
Q

⭐

A

positions r relevant to the group already attached (directing group)

a level chemistry (39 decks)

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